After a cold winter night in a mountain valley, daybreak might reveal a landscape covered in a still, milky blanket of fog. Or it might not.
This notoriously unpredictable weather phenomenon can be dangerous, especially for drivers. From 2007 to 2016, fog in the United States caused more than 25,000 crashes and contributed to 464 driving-related fatalities, according to the U.S. Department of Transportation.
One type of fog common to mountainous regions, known as cold fog, is correlated, in part, with clear skies and shifts to freezing temperatures, according to new research in Utah. The finding paves the way for scientists to better understand and predict where the hazard may pop up.
“Very little information is available about mountain fog, but it’s quite important to predict it if possible.”
“Very little information is available about mountain fog,” said Harindra Joseph Fernando, an environmental engineer at the University of Notre Dame who was not involved in the research. “But it’s quite important to predict it if possible.” Improved forecasts could give the warning needed to keep the roads clear when cold fog is likely.
Cold Fog Questions
Cold fog forms when temperatures are below freezing; tiny water droplets freeze when they come into contact with particles in the air. But conditions must be just right.
As part of a project called Cold Fog Amongst Complex Terrain, or CFACT, researchers observed nine periods of fog and nonfog conditions in Heber Valley, Utah, in January and February 2023. The valley’s typically cold nights made for good cold fog conditions.
Observation periods lasted 2 days. A team of undergraduates and postdoctoral fellows worked through the chilly, early mornings, placing low-cost weather stations and releasing weather balloons into the valley. The instruments collected data on aerosols, weather conditions, soil and air moisture and temperature, heat flow, air turbulence, droplet size, and more.
“This is the most comprehensive program to study cold fog, to our best knowledge,”said Zhaoxia Pu, an atmospheric scientist at the University of Utah. Pu was the lead author on the new research. Scientists know less about cold fog formation than coastal or warm fog conditions, as most experiments on fog in the United States have occurred along the California coast, according to the researchers.
The team identified new conditions that contribute to cold fog formation, including high atmospheric pressure and clear skies, which facilitate the cooling of air near the ground, said Fernando. The team also found that above-freezing daytime air and soil temperatures, which allow for humid air, helped form cold fog.
A period of elevated air turbulence helped fog formation by mixing air, bringing saturated surface air aloft to condense into fog, Pu said. Calm conditions after turbulence helped to keep fog in the valley.
The new research does an “excellent job” of looking at the small-scale processes that result in cold fog formation, Fernando said.
Sharpening Fog Forecasts
Forecasting fog is more difficult than forecasting other weather phenomena for a few reasons: First, fog often forms over tens to hundreds of meters, below the several-kilometer resolution of most numerical models that forecasters use. “It’s very localized,” Pu said.
“Usually, the conditions are just on the margin, just on the balance between fog or no fog.”
In addition, fog formation is extremely complex: Numerous factors, from soil temperature to humidity to air pollution from cars, can make or break a foggy day. Fog also flashes in and out of existence, making it hard to track, said Clive Dorman, a marine meteorologist at the Scripps Institution of Oceanography who was not affiliated with the study. “Usually, the conditions are just on the margin, just on the balance between fog or no fog,” he said.
Some of the many factors that contribute to fog formation are not routinely and widely measured, so basic forecasts by the National Weather Service may not have all the relevant information to accurately forecast fog. The missing details could easily throw a forecast off, he said.
The data gathered by the researchers are specific to complex terrain that has varied land use, land cover, and elevation, such as Heber Valley and the U.S. Mountain West, Fernando said. Coastal regions can also have complex terrain. But coastal fog forms differently than cold fog because of moisture provided by the ocean, meaning the study’s results are less applicable there, Pu said.
According to Fernando, the data provided by the CFACT project have the potential to improve forecasting. But to get there, the researchers must complete additional phases of the study: describing the cause-and-effect relationships between their data and fog formation, incorporating those data into numerical models that forecasters use, and validating those improved models with further observations of cold fog, he said.
—Grace van Deelen (@GVD__), Staff Writer
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